Known fiber-to-the-home (FTTH) systems send and receive bidirectional optical signals over a single fiber. Signals from a headend, generally at a wavelength of 1550 nm, are sent downstream to an ONU (which may comprises an RF over glass ONU or RONU) at a customer location, and signals from the ONU are sent upstream, generally at either 1310 nm or 1610 nm, back to the headend. Wavelength division multiplexing (WDM) methods allow these signals to travel over a single fiber at the same time. The ONU is connected to a customer premises device, such as a cable modem, a media terminal adapter or a set top box, using coaxial cable or another medium for carrying electrical radio frequency (RF) signals. The ONU converts optical signals arriving at the ONU from the headend into RF signals usable by the customer premises device using an opto-electrical (O/E) converter and converts RF signals arriving at the ONU from the customer premises device into optical signals for transport over the optical fiber using an electro-optical (E/O) converter. In this manner, the capacity of optical fibers can be enjoyed for most of the distance from the headend to the customer premises device while still using a conventional customer premises device that sends and receives RF signals.
Such networks may include optical splitters for selectively removing signals from an optical fiber and carrying them to an ONU. In a centralized system, as illustrated in FIG. 4, a main fiber 200 arrives at a splitter 202, and the signal from the main fiber 200 is split into a desired number of additional signals for transmission over additional fibers, such as fiber 204, that connects to an ONU, such as ONU 206, connected to a customer premises device, such as customer premises device 208. In a fully distributed system, illustrated in FIG. 5, the splitters may be referred to as “taps” 210 and each may, for example, split one or more signals from a main fiber 212 at a number of different locations for transport over an additional fiber, such as fiber 214, to an ONU such as ONU 216 and from there to a customer premises device such as customer premises device 218. As will be appreciated, in such systems, the distance from the headend to each customer premises device will vary. Because of the differing fiber lengths, the port-to-port variation of given optical splitter components and the cascading of this variance when a split is not centralized, the optical received power at each ONU will vary over some design range.
In a process known as RF long loop automatic gain control (AGO), a controller at the headend sends a signal to a customer premises device to set the RF output level of the customer premises device. The controller at the headend adjusts the RF output level in an effort to keep the strength of optical signals received from each ONU at the headend in the system at a given level. Thus, for example, if the strength of an optical signal received at the headend from a given ONU is too high, the headend may instruct the customer premises device connected to that ONU to transmit RF signals at a lower power. This in turn will cause the ONU to transmit optical signals having a lower power.
The headend will receive optical signals from a first ONU at a first level and signals from the a second ONU at a second level less than the first level. The difference in RF signal levels that produce these optical signals will be twice the difference in optical signal strength. This is due to the optical-to-electrical conversion properties of the optical receiver used in a network to terminate the optical signal from the FTTH premise equipment. For example, assume that the highest loss optical link in a splitter group has a loss—that is 6 dB greater than the lowest loss link in that same splitter group, and that the customer premises devices are configured to operate at an RF output of 50 dBmV. With this network configuration, the customer premises device on the shortest optical link, when operating at 50 dBmV, will produce an RF signal at the input to the ONU that will be higher by two times the optical delta between the links. This is because when photons are converted to current in the optical detector, a 1 dB increase in optical power produces a 2 dB increase in signal current. Thus a 50 dBmV input signal to the ONU on the lowest loss optical link will cause the ONU to send an optical signal to the headend that will be 6 dB too high. As a result the headend will instruct the customer premises device to reduce its RF output level by 2*6 or 12 dB, lowering the RF output level of the ONU to 50-12, or 38 dBmV.
A problem with addressing the foregoing problem by changing the RF output level of a customer premises device is that the RF input operating range of the ONU is limited by the maximum/minimum operating range of the customer premises device. The RF variance reduces that operating range by the amount of the RF variance. For example, a DOCSIS 3.0 cable modem may have a maximum RF output of 51 dBmV when operating in a 4 bonded channel mode. It may also have a minimum RF output of 23 dBmV when operating at the highest symbol rate. This makes the operating range 28 dB. If the optical variance is 6 dB, then there is 12 dB of RF variance. This reduces the RF operating range to 28-12, or 16 dB. This means that the amount of inside wire loss and system margin for the system is reduced from 28 to 16 dB.
One method to address this issue is to utilize an FM system as discussed in U.S. patent application No. 2008/0310842 to Skrobko and U.S. patent application No. 2008/0310846 to Ibelings. Such systems may allow for a single RF input level at the FTTH premise equipment because the link gain is constant and independent of the optical link loss. However, an FM system is limited because it cannot support the activation of more than one FTTH premise equipment at any given time. If that occurs, two FM signals are sent that cannot be resolved at the receiver, and both data transmissions are lost. This forces an operator to implement a single upstream system protocol, such as DOCSIS, which generally means a customer premises device upgrade/change at the customer location. This is also an issue for users who want to implement multiple services on different equipment in the serving office, but who do not want to have to synchronize the timing of those systems.
It would therefore be desirable to provide a method and device for obtaining a desired optical signal level input level at a network headend without adversely affecting the RF input range of ONU's in the system.